The present invention relates to an expandable stent and to a method for production of same.
Stents are generally known. Indeed, the term xe2x80x9cstentxe2x80x9d has been used interchangeably with terms such as xe2x80x9cintraluminal vascular graftxe2x80x9d and xe2x80x9cexpansible prosthesisxe2x80x9d. As used throughout this specification the term xe2x80x9cstentxe2x80x9d is intended to have a broad meaning and encompasses any expandable prosthetic device for implantation in a body passageway (e.g. a lumen or artery).
In the past six to eight years, the use of stents has attracted an increasing amount of attention due the potential of these devices to be used, in certain cases, as an alternative to surgery. Generally, a stent is used to obtain and maintain the patency of the body passageway while maintaining the integrity of the passageway. As used in this specification, the term xe2x80x9cbody passagewayxe2x80x9d is intended to have a broad meaning and encompasses any duct (e.g. natural or iatrogenic) within the human body and can include a member selected from the group comprising: blood vessels, respiratory ducts, gastrointestinal ducts and the like.
Stent development has evolved to the point where the vast majority of currently available stents rely on controlled plastic deformation of the entire structure of the stent at the target body passageway so that only sufficient force to maintain the patency of the body passageway is applied during expansion of the stent.
Generally, in many of these systems, a stent, in association with a balloon, is delivered to the target area of the body passageway by a catheter system. Once the stent has been properly located (for example, for intravascular implantation the target area of the vessel can be filled with a contrast medium to facilitate visualization during fluoroscopy), the balloon is expanded thereby plastically deforming the entire structure of the stent so that the latter is urged in place against the body passageway. As indicated above, the amount of force applied is at least that necessary to expand the stent (i.e., the applied the force exceeds the minimum force above which the stent material will undergo plastic deformation) while maintaining the patency of the body passageway. At this point, the balloon is deflated and withdrawn within the catheter, and is subsequently removed. Ideally, the stent will remain in place and maintain the target area of the body passageway substantially free of blockage (or narrowing).
See, for example, any of the following patents:
U.S. Pat. No. 4,733,665 (Palmaz),
U.S. Pat. No. 4,739,762 (Palmaz),
U.S. Pat. No. 4,800,882 (Gianturco),
U.S. Pat. No. 4,907,336 (Gianturco),
U.S. Pat. No. 5,035,706 (Gianturco et al.),
U.S. Pat. No. 5,037,392 (Hillstead),
U.S. Pat. No. 5,041,126 (Gianturco),
U.S. Pat. No. 5,102,417 (Palmaz),
U.S. Pat. No. 5,147,385 (Beck et al.),
U.S. Pat. No. 5,282,824 (Gianturco),
U.S. Pat. No. 5,316,023 (Palmaz et al.),
Canadian patent 1,239,755 (Wallsten), and
Canadian patent 1,245,527 (Gianturco et al.),
the contents of each of which are hereby incorporated by reference, for a discussion on previous stent designs and deployment systems.
While prior stents which are reliant on plastic deformation of the entire stent structure for deployment have achieved a certain degree of success, they do suffer from some disadvantages. One particular disadvantage is that the stent structure is susceptible to the occurrence of xe2x80x9cmicro-cracksxe2x80x9dxe2x80x94i.e., cracks of relatively small width and depthxe2x80x94especially in curved regions of the stent structure. Also, plastic deformation can lead to the occurrence of uneven thinning of the stent material. The occurrence of such cracks and/or uneven thinning can lead to weakened radial rigidity of the stent which, in turn, can lead to devastating consequences for the patient. Additionally, the many of the prior art stents are time consuming and expensive to produce.
Published International patent application WO 95/26695 [Lau et al. (Lau)] teaches a self-expandable, foldable stent which may be delivered using a catheter or other technique. The purported point of novelty in Lau relates to a stent which may be folded along its longitudinal axis. The folding is accomplished by conferring bending and twisting stresses to the stent, which stresses, for the material used to produce the stent, do not exceed that minimum stresses above which plastic deformation of the stent will occurxe2x80x94i.e., application of these stresses to the stent results in the storage of mechanical energy in the stent but does not result in the occurrence of any plastic deformation. The stent disclosed by Lau is disadvantageous since a relatively complicated folding protocol is needed. Specifically, in the illustrated embodiments, Lau teaches that the stent is folded into xe2x80x9ca loose-C configurationxe2x80x9d (FIGS. 33A and 33B), xe2x80x9ca rolled configurationxe2x80x9d (FIGS. 33C and 33D) or xe2x80x9ca triple lobed configurationxe2x80x9d (FIGS. 33E and 33F). The stent taught by Lau is also disadvantageous since, after it is folded (and thus contains the bending/twisting stresses described above), the stent must be restrained mechanically from spontaneous expansionxe2x80x94see, for example, FIGS. 35A, 35B, 36A, 36B, 39 and 40 of Lau which illustrate complicated tethering systems for mechanically restraining the folded stent from spontaneous expansion.
Published European patent application 0,669,114A [Fischell et al. (Fischell)] teaches a stent having a multiplicity of closed circular structures connected by a series of longitudinals. The stent is initially produced in a pre-deployment form comprising ovals connected by the longitudinals (see FIGS. 4 and 5). The pre-deployment form of the stent is than placed on the end of a balloon stent delivery catheter and the ovals are folded about their minor axis by securing the ovals at each end of the structure and translating a pair of opposed longitudinals (see FIG. 6). A disadvantage of this approach is that, during the folding step, plastic deformation of the stent structure unavoidable since folding is accomplished by securing selected portions of the stent while translating other portions of the stent. As discussed above, while this is disadvantageous during expansion of the stent, the disadvantages are heightened if the stent undergoes plastic deformation during adaption of the unexpanded stent to a delivery system prior to expansion. A further disadvantage of this approach is the need have distinct unfolded pre-deployment (i.e., post-production/no balloon), folded pre-deployment (i.e., on balloon) and post-deployment forms of the stent.
Accordingly, it would be desirable to have an improved stent which overcomes these disadvantages. It would be further desirable if the improved stent could be manufactured readily.
It is an object of the present invention to provide a novel expandable stent which obviates or mitigates at least one of the above-mentioned disadvantages of the prior art.
It is another object of the present invention to provide a novel method for manufacturing an expandable stent.
Accordingly, in one of its aspects, the present invention provides an expandable stent comprising a proximal end and a distal end in communication with one another, a tubular wall disposed between the proximal end and the distal end, the tubular wall having a longitudinal axis and a porous surface defined by a plurality of interconnecting struts, a series of the struts connected to one another at an interconnection point, the struts being reversibly hingable at the interconnection point between a first, contracted position and a second, expanded position, the stent being unrestrained in and expandable from the first position to the second position upon the application of a radially outward force on the stent.
In another of its aspects, the present invention provides an expandable stent comprising a proximal end and a distal end in communication with one another, a tubular wall disposed between the proximal end and the distal end, the tubular wall having a longitudinal axis and a porous surface defined by a plurality of interconnecting struts, a series of the struts connected to one another at a plastically bendable interconnection point, the struts being reversibly hingable at the plastically bendable interconnection point between a first, contracted position and a second position, expanded position, the stent being expandable from the first position to the second position upon the application of a radially outward force on the stent.
In yet another of its aspects, the present invention provides a process for producing a stent comprising a proximal end and a distal end in communication with one another, a tubular wall disposed between the proximal end and the distal end, the tubular wall having a longitudinal axis and a porous surface defined by a plurality of interconnecting struts, a series of the struts connected to one another at an interconnection point, the struts being reversibly hingable at the interconnection point between a first, unrestrained contracted position and a second, expanded position, the process comprising the step of applying a radially compressing the stent in the second position to produce the stent in first position.
In yet another of its aspects, the present invention provides a process for producing a stent comprising a proximal end and a distal end in communication with one another, a tubular wall disposed between the proximal end and the distal end, the tubular wall having a longitudinal axis and a porous surface defined by a plurality of interconnecting struts, a series of the struts connected to one another at an interconnection point, the struts being reversibly hingable at the interconnection point between a first, unrestrained, contracted position and a second position, expanded position, the process comprising the steps of:
(i) selectively removing portions of a solid tubular wall having a diameter substantially the same as that of the stent in the second position to produce a multiple-stent tube defined by multiple sections of the porous surface connected to one another by a series of radially disposed connecting members;
(ii) radially compressing the multiple-stent tube such that the multiple sections have a diameter substantially the same as that of the stent in the first position; and
(iii) removing the connecting members between the multiple sections to produce the stent in the first position.
Thus, the present inventor has developed a novel stent which is fundamentally different from stents produced heretofore. The present stent is expandable from a first, contracted position to a second, expanded position without the stent undergoing significant plastic deformation throughout its structure. Further, in the first, contracted position the present stent, unlike the stent of Lau described above, has no mechanical forces stored therein. Accordingly, a distinct advantage of the present stent over the stent of Lau is that, in the contracted state, the present stent does not have to be mechanically restrainedxe2x80x94i.e., in the present stent may be consider to be mechanically unrestrained in the contracted state. A preferred feature of the present stent is that, in the contracted state, it generally assumes the structure of a tubular wall having a substantially circular cross-section. A further preferred feature of the present stent is that it has substantially the same cross-sectional shape in the first, contracted (i.e, unexpanded) state and the second, expanded shape. When compared to the stent of Lau and Fishcell, this feature of the present stent is advantageous since it facilitates even expansion of the stent during deployment.
In the present stent, a series of the struts defining the porous surface of the stent are connected at an intersection point. This intersection point actually functions as a hinge about which the stent may be transformed from the contracted state to the expanded state or vice versa. For, example, when the stent is transformed from the contracted state to the expanded state, this generally occurs by a widening of the angle between adjacent struts at the intersection point instead of plastic deformation of the entire network of struts. Specifically, the intersection point (only) undergoes a minor plastic bending during hinging of the stent between the contracted and expanded states. However, since plastic deformation of the entire network of the struts is avoided in the present stent, the occurrence of microcracks on and/or uneven thinning of the stent surface is significantly reduced or eliminated.
Another advantage accruing from a preferred embodiment of the present stent is that, since the stent expands by hinging open of the struts at the intersection point, the change in the longitudinal dimension of the stent during expansion is substantially negligible. This advantage of the present stent is facilitated by a preferred feature of providing a compression means in one or more of the longitudinally disposed struts. If present stent, the specific shape of the compression means disposed in the longitudinal strut is not particularly restricted provided that it serves to minimize or inhibit lengthening of the stent when it is transformed from the unexpanded state to the expanded state, or vice versa. Preferably, the compression means comprises at least one lateral section disposed in the longitudinal strut, more preferably at least a first lateral section and a second lateral section disposed in the longitudinal strut. By xe2x80x9clateral sectionxe2x80x9d is meant a section of the longitudinal strut which is bowed in or out of (i.e., radially from) the strut. The apex of the lateral section may be pointed, rounded or substantially flat. When the compression means comprises a first lateral section and a second lateral section, the two sections may be symmetric or asymmetric (in the case of asymmetric this includes two sections of the same shape but different size and two sections of different and size). Further, when the compression means comprises a first lateral section and a section lateral section, the sections may be bowed in the same or opposite direction.
A particularly preferred embodiment of the compression means comprises a sinusoidal or S-shaped section (an example of such a section is illustrated herein and discussed below).
As will be described in more detail hereinbelow, the present process essentially comprises starting with a tubular wall having a porous surface defined by a plurality of interconnecting struts and thereafter hinging or compressing the tubular wall, in essence, to produce the stent in an unrestrained, contracted state. An advantage of this approach is that the desired porous surface after deployment of the stent is conferred to the tubular wall during production of the stent. A further advantage of this approach is that the present stent may be hinged between the first, contracted (i.e., unexpanded) position and the second, expanding position without the need to produce an intermediate position. Specifically, unlike the approach in Fischell of having distinct unfolded pre-deployment (i.e., post-production/no balloon), folded pre-deployment (i.e., on balloon) and post-deployment forms of the stent, as will be developed below, the present approach is initially to produce the stent in the desired final form (i.e., the second, expanded position) and thereafter radially compress the stent to a predeployment form (i.e., the first, contracted position). The advantage of the present approach is that expansion of the stent to the second, expanded position is greatly facilitated.
Thus, some of the advantages accruing from the present stent may be summarized as follows:
(i) the stent is produced initially in the desired configuration, then radially compressed (or otherwise folded) to a contracted position and finally returned to the original desired configuration via expansion by a balloon or other mechanical means thereby simplifying production and use;
(ii) in the deployed stent, the structure of the stent has not undergone any plastic deformation which obviates or mitigates the occurrence of micro-cracks and/or thinning; and
(iii) since the structure of the present stent does not undergo plastic deformation, the occurrence of recoil in the deployed stent is obviated or mitigated (this is a major advantage of the present stent).
As will be apparent below, there are also many processing advantages which accrue from the present process. These include: (i) elimination of conventional remelt removal steps, and (ii) more efficient electropolishing and radio-opaque marker (e.g. gold) deposition steps, when these steps are used in conjunction with the present process.